The present invention relates generally to a technique for reproducing optically an information signal recorded on a record medium such as a video disc, and more particularly, to a jitter correction circuit for correcting a time base error of the reproduced information signal due to a fluctuation in a rotating speed of the disc, an eccentricity of the disc, etc.
Usually, such a record medium has recorded therein an information signal which includes a first synchronizing signal of a first frequency and a second synchronizing signal of a second frequency which is higher than the first frequency of the first synchronizing signal. For instance, a video disc has recorded thereon a color television signal of NTSC system having a horizontal synchronizing signal of about 15.75 KHz and a color sub-carrier of about 3.58 MHz which is much higher than said horizontal synchronizing signal frequency.
Heretofore, it has been known to correct the time base error in an optical disc player by means of a servo control system comprising a galvanomirror which deflects a light spot on the disc in a tangential direction to an information track. However, the reliability of the galvanomirror including a rotating shaft is relatively low. In order to improve the reliability, the manufacturing cost of the galvanomirror would be increased. Further, the galvanomirror might produce acoustic noise during operation.
In the actual information reproducing head, it is necessary to provide focussing and tracking servo mechanisms for correcting focussing and tracking errors so as to cause the small light spot to follow the information track accurately. For instance, the focussing servo mechanism may be constituted by a moving coil system and the tracking servo mechanism by a galvanomirror or a device for driving an objective lens in a disc radial direction. Therefore, if the above mentioned tangential galvanomirror for correcting the time base error is provided, the head is liable to be complicated in construction, heavy in weight and expensive in cost. Under these circumstances, it is desired to effect the jitter correction by means of an electronic circuit.
It has been also proposed to compensate the time base error by means of an electronic control system comprising a delay device having an array of charge coupled devices (hereinafter referred as CCD delay device) and a voltage controlled oscillator (VCO) for generating clock pulses for driving the CCD delay device. The electronic control system may be further classified into an open loop system and a closed loop system. In the open loop system, an operational frequency range of the system can be made wider and the system can follow the jitter component in a prompt manner, but there is a problem in a linearity of the VCO for driving the CCD delay device. While in the closed loop system, the linearity of the VCO does not cause any trouble, but it is difficult to obtain a large loop gain due to the fact that the CCD delay device includes an inherent delay time. For instance, if the control is carried out with respect to a horizontal synchronizing signal of a color signal of the NTSC system reproduced from the video disc, it is only possible to limit a residual jitter up to about 80 nsec. to 100 nsec. Although the residual jitter may be further reduced by effecting the control with respect to a color burst signal of the color television signal, a relatively long time is required to draw the system into a controllable state.